The ability to detect minute amounts of protein is of interest both in protein chemistry and immunology. A possible replacement for radioimmuno assay (RIA) is the tagging of the protein by a caged molecule of Europium. By taking advantage of the very long fluorescence lifetime of Europium (500 microseconds) extreme sensitivities are possible. A pulsed nitrogen laser operating at 337 nm at 15 pps and power output of 200 microjoules been used with a 1/2 inch end of PET photomultiplier tube to examine a simple Europium tagged system.